In the production of aluminium with current electrolysis technology, based on so-called Hall-Héroult cells, the operation of the cells depends on the formation and maintenance of a protective layer of frozen electrolyte in the side lining of the cell. This frozen bath is called the side layer, and it protects the cell's side lining against chemical and mechanical wear. It is an essential condition for achieving long cell lives. The crystallized bath also functions as a buffer for the cell with regard to changes in thermal balance. During operation, the generation of heat and the thermal balance in the cell will vary as a consequence of undesired operating disturbances (changes in bath acidity, changes in aluminium concentration, changes in interpolar distance, etc.) and desired events in the cells (tapping metal, changing anodes, anodic effect, etc.). This leads to the thickness of the layer changing in the periphery of the cell, and, in some cases, the layer may disappear entirely in parts of the periphery. The side lining will then be exposed to electrolyte and metal, which, in combination with oxidizing gases, will lead to corrosion of the side lining materials with the result that they will be eroded. Over long-term operation, leakages in the side are often the result of such repeated events. It is therefore important to control layer formation and layer stability in Hall-Héroult cells. For Hall-Héroult cells with a high current density, model calculations show that it will be difficult to maintain the side layer in the cell on account of high heat generation. For such cells and for traditional cells with thermal balance problems, long cell life will therefore be subject to the ability to maintain the layer that protects the side lining.
Production of aluminium in accordance with the Hall-Héroult principle currently takes place with relatively high energy consumption measured in kilowatt hours per kilogram of aluminium. Heat is generated in an electrolysis cell as a consequence of ohmic voltage drop in the cell, for example in current leads, produced metal and, not least, in the electrolyte. Around 55% of energy supplied to the electrolysis cell is used to produce heat in the cell. Literature data indicates that approximately 40% of the total heat loss from the cells is through the side lining. On account of the high heat loss and the protective, frozen layer in the side lining, this area of the cell is an advantageous place for elements for heat recovery.
In order to optimize both of these aims simultaneously, i.e. control of layer formation and heat recovery, it is important for the heat recovery to take place as close to the side layer formed as possible. This will result in the control of and speed of layer formation being as fast as possible and the temperature difference between incoming and outgoing coolant being as high as possible. The latter is optimal for energy utilization/recovery.
The present invention concerns an improved material design and production of this in order to contribute to increased control of side layer formation and the possibility of heat recovery in aluminium electrolysis cells.
The use of heat exchange to regulate heat flow in aluminium electrolysis cells has previously been described in German patent publications, among others. Publications DE 3033710 and EP 0047227 from Alusuisse both describe this technology. The publications describe a “construction” that is embedded in the cell's side lining. Heat is conducted through this construction and on to the outside of the cell where it is exchanged with a coolant, for example based on sodium metal. This coolant and the construction of the heat exchanger are known from previous publications and are usually called heat pipes. The material used in the cooling unit is made of metal with good heat-conducting properties. To increase the effectiveness of the heat exchange, an insulating layer is inserted between the carbonaceous side lining and the steel casing of the electrolysis cell. As indicated in the two publications, one of the aims of the design is to regulate heat flow through the cell's side lining and thus control the thickness of the side layer. In addition, they refer to the invention also making it possible to operate existing cells with increased current intensity, and increases of up to 25% are suggested.
U.S. Pat. No. 4,222,841 describes a possibility for heat exchange in aluminium electrolysis cells. The patent is based on the introduction of tubular cooling ducts in the side lining and base lining and over the electrolyte. The aim of the cooling is to control the bath temperature in the electrolysis cell and make cell operation, i.e. layer formation in the side lining, more independent of the current intensity supplied to the cell. The patent does not describe which materials are to be used in the heat exchanger, but it stipulates that they must be resistant to the corrosive atmosphere in the cell and also be oxidation-resistant as air is proposed as a coolant, among other things.
WO 83/01631 refers to a device for heat exchange of hot exhaust gases from closed electrolysis cells. The heat in the exhaust gases is used to preheat the feed flow of aluminium oxide to the electrolysis cell, and the regulation of the side layer's thickness in the cell as such is not an issue.
WO 87/00211 (see also NO 86/00048) from H-Invent describes a principle and a method for heat recovery from aluminium electrolysis cells. The publication describes metal plates with spiral ducts for extraction of heat from the side lining. Various coolants can be used. Among others, helium is mentioned in particular in the patent. The hot exhaust gases from heat exchange in the side lining can be used for energy production by driving an expansion machine that, in turn, drives an electric generator. The material in the heat exchanger plates is made of metal. In order to protect these plates against liquid electrolyte, an external layer of fireproof material, for example carbon, is used against the electrolyte. One problem with this solution will be ensuring good contact between the heat exchanger plates and the external cladding of fireproof material. Poor contact between these two layers will reduce the effect of the heat exchanger installation and thus lead to reduced heat recovery and reduced control of the side layer's thickness in the electrolysis cell.
Norwegian patent applications NO 20002889, NO 20014874 and NO 20005707, international patent application WO 02/39043 and Norwegian patent NO 312770, all from Elkem Aluminium, describe a different version of the previously mentioned heat pipes for cooling aluminium electrolysis cells, among others. The patents describe heat pipes for which sodium metal is mentioned in particular as a coolant. The side walls of the electrolysis cell are thermally insulated with a fireproof material between the steel shell and an inner evaporation-cooled panel that is in contact with the electrolyte and/or the frozen side layer. The lower part of the evaporation-cooled panel contains liquid coolant that evaporates on account of the heat supplied from the electrolyte, and the upper part of the evaporation-cooled panel contains a closed cooling duct connected to an outer circuit. In this part of the evaporation-cooled panel, the coolant will condense, and heat can be extracted through the coolant, preferably various types of gas that flow through the cooling duct mentioned above. In the case of heat exchange in several stages, the heat emitted from the electrolysis cell can be used to drive an electric turbine to generate electricity. This will result in a considerable reduction in the effective electrical energy consumption in the electrolysis cell per tonne of aluminium produced. The patent (NO 312770) states that the evaporation-cooled panels should preferably be made of non-magnetic steel. A possible problem of this patent is associated with the difficulties of producing a corrosion-resistant steel that will function in an atmosphere consisting of oxygen and fluorides at around 1000° C. It is known from the literature that the presence of fluorides at elevated temperatures produces a dramatic increase in the oxidation rate of steel.